Silicon/germanium oxide particle inks, inkjet printing and processes for doping semiconductor substrates

ABSTRACT

Highly uniform silica nanoparticles can be formed into stable dispersions with a desirable small secondary particle size. The silican particles can be surface modified to form the dispersions. The silica nanoparticles can be doped to change the particle properties and/or to provide dopant for subsequent transfer to other materials. The dispersions can be printed as an ink for appropriate applications. The dispersions can be used to selectively dope semiconductor materials such as for the formation of photovoltaic cells or for the formation of printed electronic circuits.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationSer. No. 60/878,239 filed on Jan. 3, 2007 to Hieslmair et al., entitled“Doped Dispersions and Processes for Doping Semiconductor Substrates,”incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to dispersions/inks of silica/germania particles,such as doped silica particles. The invention further relates to inksthat are suitable for ink jet printing. Additionally, the inventionrelates to the use of doped silica/germania particles for the doping ofsemiconductor substrates, such as through the drive in of dopants withheat and/or light from a silica/germania deposit formed through inkjetprinting onto the semiconductor surface.

BACKGROUND OF THE INVENTION

The formation of semiconductor devices generally involves the formationof doped regions in which the dopants alter the electrical conductionproperties or other desired properties. Through the selected dopingprocess different domains can be formed in the material that providefunctionalities for particular semiconductor devices. For example, somedopants provide excess electrons that can populate the conduction bands,and the resulting materials are referred to as n-type semiconductors.Other dopants provide holes and are used to form p-type semiconductors.Additional dopants can provide other functionalities, such as opticalemissions. Through appropriate doping, a wide range of devices can beformed, such as transistors, diodes and the like.

With increasing costs and undesirable environmental effects from the useof fossil fuels and other non-renewable energy sources, there aregrowing demands for alternative forms of energy. Various technologiesare available for the formation of photovoltaic cells, i.e., solarcells. A majority of commercial photovoltaic cells are based on silicon.Increased commercialization of alternative energy sources relies onincreasing cost effectiveness through lower costs per energy unit. Thus,for a photovoltaic cell, the objective would be to increase energyconversion efficiency for a given light fluence and/or to lower the costof producing a cell.

SUMMARY OF THE INVENTION

In a first aspect, the invention relates to a dopant ink compositionsuitable for ink jet printing, the ink comprising a stable dispersion ofsilica/germania nanoparticles in a dispersing liquid with aconcentration of silica/germania nanoparticles from about 0.1 weightpercent to about 20 weight percent. The silica/germania nanoparticlescan have an average primary particle size of no more than about 100 nm,and the dopant ink generally has a viscosity from about 0.1 mPa·s toabout 100 mPa·s.

In a further aspect, the invention relates to a dispersion comprising aliquid and doped silica/germania nanoparticles. In some embodiments, thedoped silica/germania nanoparticles having an average primary particlesize from about 1 nm to about 100 nm and a volume-average particle sizeof no more than about 500 nm. The dispersion can have a concentration ofdoped silica/germania nanoparticle from about 0.1 weight percent toabout 20 weight percent doped silica/germania nanoparticles. Thesilica/germania nanoparticles can comprise a dopant element from about1.0×10⁻⁷ to about 15 atomic percent relative to the silicon atoms of thesilica/germania nanoparticles.

In another aspect, the invention relates to a method for forming adispersion in which the method comprises blending a collection of dopedsilica/germania nanoparticles with a liquid and the liquid andnanoparticles are selected to be compatible with each other. Thenanoparticles have an average primary particle size from about 1 nm toabout 100 nm and a volume-average secondary particle size no more thanabout 500 nm following mixing. In some embodiments, the silica/germaniananoparticles comprise a dopant element from about 1.0×10⁻⁷ to about 15atomic percent relative to the silicon/germanium atoms of thesilica/germania nanoparticles.

In other aspects, the invention relates to a method for forming adeposit of silica/germania particles on a substrate surface in which themethod comprises inkjet printing a pattern of an ink. The ink generallycomprises silica/germania particles having an average primary particlesize from about 1 nm to about 100 nm and a volume-average secondaryparticle size no more than about 500 nm.

Moreover, the invention pertains to a method for doping a semiconductorsubstrate comprising silicon or germanium in which the method comprisesheating a deposit in contact with substrate to drive dopant into thesubstrate. The deposit generally comprises doped silica/germaniaparticles in which the doped silica/germania particles comprise a dopantelement from about 1.0×10⁻⁷ to about 15 atomic percent relative to thesilicon/germanium atoms of the silica/germania particles. The particlescan have an average primary particle size from about 1 nm to about 100nm and an average second particle size no more than about 500 nm basedon a dispersion formed with the particles prior to forming a coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of a printed substrate.

FIG. 2 is an x-ray micrograph of silica particles synthesized usinglaser pyrolysis with an aerosol tetraethoxysilane precursor in which themicrograph is taken at a first magnification.

FIG. 3 is an x-ray micrograph of the sample of FIG. 2 taken at a greatermagnification.

FIG. 4 is a plot of the size distribution of a dispersion of silicananoparticles in methylethylketone liquid using dynamic light scatteringto make the measurement.

FIG. 5 is a plot of the size distribution of a dispersion of silicananoparticles in isopropanol liquid using dynamic light scattering tomake the measurement.

DETAILED DESCRIPTION OF THE INVENTION

Versatile processes for the delivery of silica and/or germaniananoparticles can be based upon the ability to form high qualitydispersions of silica nanoparticles along with the ability to producesilica nanoparticles with desirable properties. In some embodiments, thesilica particles have selected dopants, which can provide dopantelements for appropriate applications. As described herein, thedispersions of the silica particles generally have highly uniform silicaparticles that can be dispersed well such that the resulting dispersionshave a small secondary particle size with high uniformity. Since thedispersions can be formed with appropriate properties, the dispersionscan be used to formulate inks suitable for inkjet printing, whichprovides for efficient and precise deposition of the inks. Inembodiments relating to doped silica particles, the dispersions and inkscan be used to place deposits of doped silica at selected locationsalong a semiconductor substrate. The dopants from the silica particlescan be driven into the semiconductor materials to correspondingly dopethe semiconductors at the selected locations.

Silica, i.e., silicon dioxide or SiO₂, is an important material for arange of applications. For example, silica can be used to formtransparent optical materials useful for the transmission of light. Inaddition, silica is a useful dielectric for applications in electronics.Silica can be used as a dielectric with silicon based semiconductorssince silica particles can be produced such that the material does notprovide any contaminants that can negatively impact the performance of asilicon based semiconductor if the contaminants migrate into the siliconbased semiconductor. Also, silica can be selectively doped to influencethe properties of the silica. As described further below, the dopedsilica particles can be used to deliver dopants to a silicon basedsemiconductor without providing undesirable contaminants. Germania canbe used similarly with respect to germanium-based semiconductors assilica is used for silicon-based semiconductors. For some applications,mixtures or blends of silica and germania may be useful. To simplify thediscussion in the detailed description of the invention, the term silicais used to refer to silica, germania, combinations, e.g., alloys,thereof and mixtures thereof, unless otherwise stated or otherwiseclearly limited in the context, such as with respect to the descriptionof chemical precursors. With respect to the claims, silica shall referonly to silica, and silica/germania shall refer to silica, germania,combinations/alloys thereof and mixtures thereof.

Dispersions of particular interest comprise a dispersing liquid andsilica particles dispersed within the liquid along with optionaladditives. The dispersion can be stable with respect to avoidance ofsettling over a reasonable period of time, generally at least an hour,without further mixing. A dispersion can be used as an ink, i.e., thedispersion can be printed. The properties of the ink can be adjustedbased on the particular printing method. For example, in someembodiments, the viscosity of the ink is adjusted for the particularuse, such as ink jet printing, and particle concentrations and additivesprovide some parameters to adjust the viscosity. The availability toform stable dispersions with small secondary particle sizes provides theability to form certain inks, such as inkjet inks, that could not havebeen reasonably formed previously without the ability to form stabledispersions with desired properties. Other additives can be used toformulate a range of inks with appropriate properties.

The ability to form good dispersion is related to the ability tosynthesize silica particles with appropriate particle propertiesrelating to the particle size, uniformity and surface chemistry. Flowbased synthesis methods have been discovered to be very versatile withrespect to synthesizing desired silica particles. In flow based methods,such as laser pyrolysis and flame pyrolysis, the product particles arecollected as a powder. Since the powder needs to be dispersed to placethe particles into a liquid, there are inherent disadvantages withrespect to solution based approaches. However, these disadvantages arecompensated by the ability to manipulate the particles during theflow-based synthesis to achieve desired particle properties. Forexample, the flow-based approaches are particularly versatile withrespect to forming doped silica particles.

Furthermore, it is desirable for the silica particles to be uniform withrespect to particle size and other properties. Specifically, it isdesirable for the particles to have a uniform primary particle size andfor the primary particles to be substantially unfused. Then, theparticles generally can be dispersed to yield a smaller more uniformsecondary particle size in the dispersion. Secondary particle sizerefers to measurements of particle size within a dispersion. Theformation of a good dispersion with a small secondary particle size canbe facilitated through the matching of the surface chemistry of theparticles with the properties of the dispersing liquid. As describedbelow, the surface chemistry of particles can be influenced duringsynthesis of the particles as well as following collection of theparticles. For example, the formation of dispersions with polar solventsis facilitated if the particles have polar groups on the particlesurface.

In general, desirable silica particles for dispersions and correspondinginks are submicron. In other words, the particle collections have anaverage primary particle diameter of no more than about 1 micron, whichis equal to 1000 nanometers (nm), and in some embodiments the averageprimary particle diameter is no more than about 100 nm. In someembodiments, the particles are very uniform in particle size. Also, theparticle can have appropriate surface properties to form gooddispersions for inks and other uses. Suitable submicron particles can beformed, for example, by vapor-based flow processes, such as flamepyrolysis or laser pyrolysis, or solution-based approaches, such as solgel approaches. Laser pyrolysis is a desirable approach for theformation of the particles since the resulting particle are generallyhighly uniform and can be well dispersed using techniques that have beendeveloped for dispersing inorganic oxide particles, as described furtherbelow. In addition, laser pyrolysis is also desirable due to itsflexibility with respect to product particle composition, such as a widerange of ability to introduce dopants.

As described herein, improved approaches have been found to disperse drynanoparticle powders, perform surface modification of the particles in adispersion and form inks and the like for deposition. Using one or moreof the improved processing approaches described herein, inks can beformed that can be deposited using inkjet printing and other convenientprinting approaches. Thus, the advantages of vapor-based particlesynthesis can be combined with desirable solution based processingapproaches with highly dispersed particles to obtain improveddispersions and inks with properties that were previously unachievable.

In some embodiments of particular interest, the particles aresynthesized by laser pyrolysis in which light from an intense lightsource drives the reaction to form the particles. Laser pyrolysis isuseful in the formation of particles that are highly uniform incomposition and size. The ability to introduce a range of precursorcompositions facilitates the formation of silica particles with selecteddopants, which can be introduced at relatively high concentrations.

Laser pyrolysis has been successfully used for the synthesis of a widerange of complex inorganic particles, including, for example,compositions with multiple metal/metalloid elements as well as dopedmaterials. For example, the synthesis of highly uniform silica particlesis described in U.S. Pat. No. 6,471,930 to Kambe et al., entitled“Silicon Oxide Particles,” and U.S. Pat. No. 6,726,990 to Kumar et al.,entitled “Silicon Oxide Particles,” both of which are incorporatedherein by reference. A wide range of dopants for introduction intosilica particles is described, for example, in U.S. Pat. No. 6,849,334to Horne et al., entitled “Optical Materials and Optical Devices,”incorporated herein by reference.

In the laser pyrolysis process, the dopant element(s) are introducedinto the reactant stream such that the elements can be incorporated intothe product particles. The dopant elements can be delivered into thereactant stream as a suitable composition. The reactant stream cancomprise vapor precursor and/or aerosol precursors. For the doping ofsemiconductor substrates, desirable dopants include, for example, B, P,Al, Ga, As, Sb and combinations thereof.

With respect to silica dispersions, the dispersions can haveconcentrations from low concentrations to about 30 weight percent. Ingeneral, the secondary particles size can be expressed as a cumulantmean, or Z-average particle size as measured with dynamic lightscattering (DLS). The Z-average particle size is an intensity averagebased on the scattered light measurements, and the scattering intensitybased particle size distribution can be converted to volumedistributions that can be used to evaluate a volume-average size.Generally, the volume-average particle size is no more than about 2microns and in some embodiments, no more than about 250 nm.Additionally, in some embodiments it is desirable for the secondaryparticle size distribution to be narrow.

In general, the surface chemistry of the particles influences theprocess of forming the dispersion. In particular, it is easier todisperse the particles to form smaller secondary particle sizes if thedispersing liquid and the particle surfaces are compatible chemically.In some embodiments, the liquid may be selected for to be appropriatefor the particular use of the dispersion, such as for a printingprocess. The surface properties of the particles can be correspondinglybe adjusted for the dispersion.

In general, the surface chemistry of the particles can be influenced bythe synthesis approach. The surface by its nature represents atermination of the underlying solid state structure of the particle.This termination of the surface of the silica particles can involve theformation of single bonds, such as Si—O—H, or strained bonds, such asSi—O—Si that introduce surface strain along the surface. The terminationof particular particles influences the surface chemistry of theparticles. The nature of the reactants, reaction conditions, andby-products during particle synthesis influences the surface chemistryof the particles collected as a powder during flow reactions.

The stability of particle dispersions can be improved at higherconcentrations of particles through surface modification of theparticles. In general, the surface properties of the particles influencethe dispersion of the particles. The surface properties of the particlesgenerally depend on the synthesis approach as well as the post synthesisprocessing. Some surface active agents, such as many surfactants, actthrough non-bonding interactions with the particle surfaces. In someembodiments, desirable properties are obtained through the use ofsurface modification agents that chemically bond to the particlesurface. Suitable surface modification agents include, for example,alkoxysilanes, which chemically bond to metal oxide and metalloid oxideparticles. For silica particles, the alkoxysilanes bond through an O—Sibond. In particular, trialkoxysilanes can form three bonds with theparticle surface, which stabilizes the bonds with the particle. Thefourth side group of the trialkoxysilane that does not bond with theparticle surface influences the resulting properties of the surfacemodified particles that relate with the interaction with fluids.

When processing a dry, as-synthesized powder, it has been found thatforming a good dispersion of the particle prior to surface modificationinvolving chemical bonding facilitates the surface modification processand results in particles with a higher degree of surface modification.The dispersion of the as-synthesized particles generally comprises theselection of a solvent that is relatively compatible with the particlesbased on the surface chemistry of the particles. Shear, sonication orother appropriate mixing conditions can be applied to facilitate theformation of the dispersion. In general, it is desirable for theparticles to be well dispersed, although the particles do not need to bestably dispersed if the particles are subsequently surface modified witha chemically bonded composition.

If the particles are dispersed prior to performing the surfacemodification, the surface modification agent can be added to thedispersion in a solvent that is soluble or miscible with dispersingliquid of the particle dispersion. Appropriate mixing can be applied tocombine the surface modifying composition with the dispersed particles.The surface modifying compound can chemically bond with the dispersedparticles to form a dispersion of surface modified particles.

The silica inks can be deposited onto the substrate using any suitableprocess, such as a selected coating process, for example, spray coating,and/or printing. While various printing approaches can be effective,inkjet printing is desirable in some embodiments due to resolution andspeed that can be achieved. Suitable inkjet heads are availablecommercially or can be constructed in simple forms using basictechnology, although new designs can be used that are more suitable fora particular application.

Printing processes can effectively coat large areas very quickly andefficiently. For semiconductor applications, the use of the printingtechniques described herein can eliminate one or more photolithographysteps. For semiconductor doping, the substrate can comprise silicon,germanium or alloy thereof. A printing process can deposit the ink atspecific locations along the substrate surface. This allows theselective doping of the substrate. Similarly, for other applicationssuch as optical applications, the printing at selected locations can beuseful for forming an optical structure with patterned optical material,which can be useful for making displays or the like. Other patterningapproaches can be used alone or combined with a printing approach, or nopatterning can be used, as appropriate for a specific application.

For semiconductor doping applications, once a doped silica ink isdeposited at selected locations, the dopant can be driven into thesubstrate to dope the surface layer of the substrate. This can be doneby heating the substrate in an oven to relatively high temperatures,such as 750° C. to 1100° C. to obtain the desired diffusion profile forthe dopant. Nanoparticles melt or flow at lower temperatures than largerparticles so that the use of nanoparticles can facilitate the dopingprocess with oven based heating. A faster process involves rapid thermalanneal in which higher temperatures of 1000° C. to 1250° C. are reachedfor short times, such as from about 20 seconds to about 120 seconds.These temperatures are still below the melting point of silicon, so thatrapid thermal annealing is still a solid state diffusion process.However, improved control of the resulting doped substrate as well asenergy saving can be obtained through the application of light energy,such as from a UV light source, using a laser or incoherent lightsource, to melt just the surface of the substrate without generallyheating the substrate or only heating the substrate to lowertemperatures. After the dopant is driven into the substrate surface, theresidual oxide material can be etched off of the substrate, such asusing wet etching or plasma etching.

In general, the doping approaches described herein can be applied to anyappropriate semiconductor processing applications. Thus, the dopantapproaches described herein can be used in the formation of integratedcircuits and other electrical and electro-optical devices, such asmicroelectromechanical (MEMS) devices. In particular, these processingapproaches can be effectively used in the formation of photovoltaicdevices, i.e., solar cells. The processes herein are well suited forlarge area semiconductors such as photovoltaic panels as well as otherlarge area applications, such as optical applications. Similarly,through the printing process the doped silica can be used to deliverdopants to corresponding select locations on the semiconductor surfaceso that printed electronics can be formed. Printed electronics can be aneffective approach for the formation of semiconductor devices withmoderate resolution of electrical components at a lower cost thanphotolithography.

For solar cell applications, the use of doped silica particles to dopecorresponding sections of silicon semiconductors has been known for sometime. Screen printing of doped silica sol-gel solution to dope asemiconductor for a solar cell is described in U.S. Pat. No. 4,104,091to Evans, Jr. et al., entitled “Application of Semiconductor Diffusantsto Solar Cells by Screen Printing,” incorporated herein by reference.Once the silica is printed to place the silica at desired locations, theprinted structure is heated to drive the dopants form the silica intothe adjacent semiconductor.

The formation of desirable solar cell elements on a thin silicon foil isdescribed further in copending U.S. Provisional patent application60/902,006 to Hieslmair et al. filed Feb. 16, 2007, entitled“Photovoltaic Cell Structures, Solar Panels and CorrespondingStructures,” incorporated herein by reference. This provisionalapplication describes the desirability in some embodiments of formingshallow doped regions. These shallow doped regions can be convenientlyformed by printing the doped silica and using laser light to drive inthe dopants, as described further below. Although in other embodiments,thicker junctions may be desired.

Optical applications of particular interest include, for example,displays in which display elements can be formed through the printingprocess and subsequent processing to form the display element. Forexample, phosphor particles can be coated with silica to stabilize thephosphor properties. The resulting silica coated particles can bedispersed as described herein to deposit the phosphor particles. Thedeposited phosphor particles can be heated to fuse the particles into amass that forms a display element. Boron, phosphorous and/or germaniumdopants can be used to lower the flow temperature of the silicaparticles. Also, rare earth dopants can be added into phosphors to formactive materials that can emit light under appropriate activatingconditions. Thus, the printing process can be effective to form displaystructures with desired optical material at selected locations withinthe structure in which the optical materials incorporate particulardopants to influence the optical and/or physical properties of thematerial.

In other applications, the particles can be deposited to form waveguidesand the like, which can be useful for optical communicationsapplications. The silica dopants can be selected to adjust theindex-of-refraction of the particles. The index-of-refraction of theresulting optical structure following fusing of the particles can bealtered through the adjustment of the index-of-refraction of theparticles. The index-of-refraction of the waveguides is selected toprovide for total internal reflection of selected wavelengths of light.The waveguide can be printed to follow a selected optical pathway.

As described herein, high quality dispersions of silica, with or withoutdopants, provides the ability for effective printing of the silica toform higher resolution structures. Due to the enhanced ability tocontrol the properties of the inks, the silica can be printed rapidlyand with relatively high resolution, for example, using inkjet printingor other desired approach. The ability to introduce selected dopantsover a wide range of compositions provides the ability to form acorresponding wide range of devices based on the silica particles. Thedopants can be driven into an underlying semiconductor substrate orincorporated into a silica structure for optical or other applications.

Particle Synthesis and Properties

The improved dispersions/inks described herein are based in part on theability to form highly uniform silica nanoparticles. Laser pyrolysis isa particularly suitable approach for the synthesis of highly uniformsilica particles. Also, laser pyrolysis is a versatile approach for theintroduction of desired dopants at a selected concentration. Also, thesurface properties of the silica particles can be influences by thelaser pyrolysis process, although the surface properties can be furthermanipulated after synthesis to form desired dispersions. Small anduniform silica particles can provide processing advantages with respectto forming dispersions/inks.

In some embodiments, the particles have an average diameter of no morethan about one micron, and in further embodiments it is desirable tohave particles with smaller particle sizes to introduce desiredproperties. For example, nanoparticles with a small enough averageparticle size are observed to melt at lower temperatures than bulkmaterial, which can be advantageous in some contexts. Also, the smallparticle sizes provide for the formation of inks with desirableproperties, which can be particularly advantageous for inkjet printingsince larger particles may clog some inkjet heads. In the formation ofoptical material, for example, the optical properties, such asindex-of-refraction and/or emission, of the silica particles can beselected through the addition of dopants. With respect to transparency,the particle size and uniformity of the particles influences thecorresponding properties of the particles when delivered appropriatelysuch as in a polymer composite.

Suitable submicron and nano-scale particles can be formed, for example,by laser pyrolysis, flame synthesis, combustion, or solution-basedprocesses, such as sol gel approaches. While laser pyrolysis is adesirable approach for particle production, submicron particles can beproduced using a flame production apparatus such as the apparatusdescribed in U.S. Pat. No. 5,447,708 to Helble et al., entitled“Apparatus for Producing Nanoscale Ceramic Particles,” incorporatedherein by reference. Furthermore, submicron particles can be producedwith a thermal reaction chamber such as the apparatus described in U.S.Pat. No. 4,842,832 to Inoue et al., “Ultrafine Spherical Particles ofMetal Oxide and a Method for the Production Thereof,” incorporatedherein by reference.

In particular, laser pyrolysis is useful in the formation of particlesthat are highly uniform in composition, crystallinity, when appropriate,and size. Laser pyrolysis involves light from an intense light sourcethat drives the reaction to form the particles. Due to the versatilityof laser pyrolysis as an excellent approach for efficiently producing awide range of nanoscale particles with a selected composition and anarrow distribution of average particle diameters, laser pyrolysis canbe used to form doped silica particles with a wide range of selecteddopants or combinations of dopants. For convenience, light-basedpyrolysis is referred to as laser pyrolysis since this terminologyreflects the convenience of lasers as a radiation source and is aconventional term in the art. Laser pyrolysis approaches discussedherein incorporate a reactant flow that can involve gases, vapors,aerosols or combinations thereof to introduce desired elements into theflow stream. The versatility of generating a reactant stream with gases,vapor and/or aerosol precursors provides for the generation of particleswith a wide range of potential compositions.

A collection of submicron/nanoscale particles may have an averagediameter for the primary particles of less than about 500 nm, in someembodiments from about 2 nm to about 100 nm, alternatively from about 2nm to about 75 nm, or from about 2 nm to about 50 nm. A person ofordinary skill in the art will recognize that other ranges within thesespecific ranges are covered by the disclosure herein. Primary particlediameters are evaluated by transmission electron microscopy.

As used herein, the term “particles” refer to physical particles, whichare unfused, so that any fused primary particles are considered as anaggregate, i.e. a physical particle. For particles formed by laserpyrolysis, the particles can be generally effectively the same as theprimary particles, i.e., the primary structural element within thematerial. Thus, the ranges of average primary particle sizes above canalso be used with respect to the particle sizes. If there is hard fusingof some primary particles, these hard fused primary particles formcorrespondingly larger physical particles. The primary particles canhave a roughly spherical gross appearance, or they can have rod shapes,plate shapes or other non-spherical shapes. Upon closer examination,crystalline particles may have facets corresponding to the underlyingcrystal lattice. Amorphous particles generally have a spherical aspect.Diameter measurements on particles with asymmetries are based on anaverage of length measurements along the principle axes of the particle.

Because of their small size, the particles tend to form looseagglomerates due to van der Waals and other electromagnetic forcesbetween nearby particles. Even though the particles may form looseagglomerates, the nanometer scale of the particles is clearly observablein transmission electron micrographs of the particles. The particlesgenerally have a surface area corresponding to particles on a nanometerscale as observed in the micrographs. Furthermore, the particles canmanifest unique properties due to their small size and large surfacearea per weight of material. For example, the absorption spectrum ofcrystalline, nanoscale TiO₂ particles is shifted into the ultraviolet.These loose agglomerates can be dispersed in a liquid to a significantdegree, and in some embodiments approximately completely to formdispersed primary particles.

The particles can have a high degree of uniformity in size. Laserpyrolysis generally results in particles having a very narrow range ofparticle diameters. Furthermore, heat processing under suitably mildconditions generally does not significantly alter the very narrow rangeof particle diameters. With aerosol delivery of reactants for laserpyrolysis, the distribution of particle diameters is particularlysensitive to the reaction conditions. Nevertheless, if the reactionconditions are properly controlled, a very narrow distribution ofparticle diameters can be obtained with an aerosol delivery system. Asdetermined from examination of transmission electron micrographs, theparticles generally have a distribution in sizes such that at leastabout 95 percent, and in some embodiments 99 percent, of the particleshave a diameter greater than about 35 percent of the average diameterand less than about 280 percent of the average diameter. In additionalembodiments, the particles generally have a distribution in sizes suchthat at least about 95 percent, and in some embodiments 99 percent, ofthe particles have a diameter greater than about 40 percent of theaverage diameter and less than about 250 percent of the averagediameter. In further embodiments, the particles have a distribution ofdiameters such that at least about 95 percent, and in some embodiments99 percent, of the particles have a diameter greater than about 60percent of the average diameter and less than about 200 percent of theaverage diameter. A person of ordinary skill in the art will recognizethat other ranges of uniformity within these specific ranges arecontemplated and are within the present disclosure.

Furthermore, in some embodiments essentially no particles have anaverage diameter greater than about 5 times the average diameter, inother embodiments about 4 times the average diameter, in furtherembodiments 3 times the average diameter, and in additional embodiments2 times the average diameter. In other words, the particle sizedistribution effectively does not have a tail indicative of a smallnumber of particles with significantly larger sizes. This is a result ofthe small reaction region to form the inorganic particles andcorresponding rapid quench of the inorganic particles. An effective cutoff in the tail of the size distribution indicates that there are lessthan about 1 particle in 10⁶ has a diameter greater than a specified cutoff value above the average diameter. High particle uniformity can beexploited in a variety of applications.

In addition, the submicron particles may have a very high purity level.Furthermore, crystalline nanoparticles, such as those produced by laserpyrolysis, can have a high degree of crystallinity. Similarly, thecrystalline nanoparticles produced by laser pyrolysis can besubsequently heat processed to improve and/or modify the degree ofcrystallinity and/or the particular crystal structure. Impurities on thesurface of the particles may be removed by heating the particles toachieve not only high crystalline purity but high purity overall.

The size of the dispersed particles can be referred to as the secondaryparticle size. The primary particle size, of course, is the lower limitof the secondary particle size for a particular collection of particles,so that the average secondary particle size can be approximately theaverage primary particle size if the primary particles are substantiallyunfused and if the particles are effectively completely dispersed in theliquid.

The secondary or agglomerated particle size may depend on the subsequentprocessing of the particles following their initial formation and thecomposition and structure of the particles. In particular, the particlesurface chemistry, properties of the dispersant, the application ofdisruptive forces, such as shear or sonic forces, and the like caninfluence the efficiency of fully dispersing the particles. Ranges ofvalues of average secondary particle sizes are presented below withrespect to the description of dispersions. Secondary particles sizeswithin a liquid dispersion can be measured by established approaches,such as dynamic light scattering. Suitable particle size analyzersinclude, for example, a Microtrac UPA instrument from Honeywell based ondynamic light scattering, a Horiba Particle Size Analyzer from Horiba,Japan and ZetaSizer Series of instruments from Malvern based on PhotonCorrelation Spectroscopy. The principles of dynamic light scattering forparticle size measurements in liquids are well established.

A basic feature of successful application of laser pyrolysis for theproduction of desirable silica particles is the generation of a reactantstream containing one or more silicon precursor compounds and, in someembodiments, a radiation absorber and/or a secondary reactant. Thesecondary reactant can be a source of oxygen atoms for the silicaproduct particles and/or can be an oxidizing or reducing agent to drivea desired product formation. A secondary reactant may not be used if theprecursor decomposes to the desired product under intense lightradiation. Similarly, a separate radiation absorber may not be used ifthe silicon precursor and/or the secondary reactant absorb theappropriate light radiation to drive the reaction. Dopant precursors canbe introduced into the reactant flow for incorporation into the silicaparticles.

The reaction of the reactant stream is driven by an intense radiationbeam, such as a light beam, e.g., a laser beam. In some embodiments, CO₂lasers can be effectively used. As the reactant stream leaves theradiation beam, the inorganic particles are rapidly quenched withparticles in present in the resulting product particle stream, which isa continuation of the reactant stream. The concept of a stream has itsconventional meaning of a flow originating from one location and endingat another location with movement of mass between the two points, asdistinguished from movement in a mixing configuration.

A laser pyrolysis apparatus suitable for the production of commercialquantities of particles by laser pyrolysis has been developed using areactant inlet that is significantly elongated in a direction along thepath of the laser beam. This high capacity laser pyrolysis apparatus,e.g., 1 kilogram or more per hour, is described in U.S. Pat. No.5,958,348, entitled “Efficient Production Of Particles By ChemicalReaction,” incorporated herein by reference. Approaches for the deliveryof aerosol precursors for commercial production of particles by laserpyrolysis is described in copending and commonly assigned U.S. Pat. No.6,193,936 to Gardner et al., entitled “Reactant Delivery Apparatus,”incorporated herein by reference. With respect to combined vapor andaerosol delivery approaches, a silicon precursor can be delivered as avapor, while one or more dopant precursors are delivered as an aerosol.However, for many desirable dopants, suitable dopant precursors can bedelivered as a vapor.

In general, nanoparticles produced by laser pyrolysis can be subjectedto additional processing to alter the nature of the particles, such asthe composition and/or the crystallinity. For example, the nanoparticlescan be subjected to heat processing in a gas atmosphere prior to use.Under suitably mild conditions, heat processing is effective to modifythe characteristics of the particles, such as removal of carboncontaminants, without destroying the nanoscale size or the narrowparticle size distribution of the initial particles. For example, heatprocessing of submicron vanadium oxide particles is described in U.S.Pat. No. 5,989,514 to Bi et al., entitled “Processing Of Vanadium OxideParticles With Heat,” incorporated herein by reference.

A wide range of simple and complex submicron and/or nanoscale particleshave been produced by laser pyrolysis with or without additional heatprocessing. Specifically, the inorganic particles can include, forexample, elemental metal or elemental metalloid, i.e. un-ionizedelements such as silver or silicon, metal/metalloid oxides,metal/metalloid nitrides, metal/metalloid carbides, metal/metalloidsulfides or combinations thereof. In addition, uniformity of these highquality materials can be substantial. These particles generally can havea very narrow particle size distribution. Laser pyrolysis methods anduniform silica particles formed by laser pyrolysis are described furtherin U.S. Pat. No. 6,471,903 to Kambe et al., entitled “Silicon OxideParticles,” and U.S. Pat. No. 6,726,990 to Kumar et al., entitled“Silicon Oxide Particles,” both of which are incorporated herein byreference. Doped silica particles formed using laser pyrolysis processare described in U.S. Pat. No. 6,849,334 to Home et al., entitled“Optical Materials and Optical Devices,” incorporated herein byreference. The production of a range of particles by laser pyrolysis isdescribed further in published U.S. Patent Application 2003/203205A toBi et al., entitled “Nanoparticle Production and CorrespondingStructures,” incorporated herein by reference.

Submicron and nanoscale particles can be produced with selected dopantsusing laser pyrolysis and other flowing reactor systems. Dopants can beintroduced at desired concentrations by varying the composition of thereactant stream. A dopant element or a combination of dopant elementsare introduced into the silica host material by appropriately selectingthe composition in the reactant stream and the processing conditions.Thus, submicron particles incorporating selected dopants, including, forexample, rare earth dopants and/or complex blends of dopantcompositions, can be formed. Generally, an oxygen source should also bepresent in the reactant stream since the dopant elements are introducedinto the silica particles as an oxide of the dopant element. In otherwords, the conditions in the reactor should be sufficiently oxidizing toproduce the oxide compositions. The doped particles can be eitheramorphous solid state blends with the dopant composition dissolved inthe host material or the dopant can be an intercalation composition. Insome embodiments, one or more dopants can be introduced inconcentrations in the particles from about 1.0×10⁻⁷ to about 15 atomicpercent relative to the silicon atoms, in further embodiments from about1.0×10⁻⁵ to about 5.0 atomic percent and in further embodiments fromabout 1×10⁻⁴ to about 1.0 atomic percent relative to the silicon atoms.A person of ordinary skill in the art will recognize that additionalranges within the explicit dopant level ranges are contemplated and arewithin the present disclosure.

Dopants can be introduced to vary properties of the resulting particles.For example, dopants can be introduced to change the optical propertiesof the particles, such as the index-of-refraction, or dopants canintroduce fluorescent or phosphorescent properties to the particles suchthat they can function as phosphors. Dopants can also be introduced toalter the processing properties of the material, such as by lowering thesoftening temperature. Furthermore, dopants can also interact within thematerials. For example, some dopants are introduced to increase thesolubility of other dopants. Also, as described below, the dopants canbe used to transfer dopants to an adjacent material, such as asemiconductor substrate.

In some embodiments, the one or plurality of dopants are rare earthmetals or rare earth metals with one or more other dopant elements. Rareearth metals comprise the transition metals of the group IIIb of theperiodic table. Specifically, the rare earth elements comprise Sc, Y andthe Lanthanide series. Other suitable dopants comprise elements of theactinide series. For optical glasses, the rare earth metals ofparticular interest as dopants comprise, for example, Ho, Eu, Ce, Th,Dy, Er, Yb, Nd, La, Y, Pr and Tm. Generally, the rare earth ions ofinterest have a +3 ionization state, although Eu⁺² and Ce⁺⁴ are also ofinterest. Rare earth dopants can influence the optical absorptionproperties that can enable the application of the materials for theproduction of optical amplifiers and other optical devices. Suitablenon-rare earth dopants for various purposes include, for example, Bi,Sb, Zr, Pb, Li, Na, K, Ba, B, Si, Ge, W, Ca, Cr, Ga, Al, Mg, Sr, Zn, Ti,Ta, Nb, Mo, Th, Cd and Sn.

In addition, dopants can be introduced to change the electricalproperties of the particles. In particular, As, Sb and/or P dopants canbe introduced into the silicon particles to form n-type semiconductingmaterials in which the dopant provide excess electrons to populate theconduction bands, and B, Al, Ga and/or In can be introduced to formp-type semiconducting materials in which the dopants supply holes. Ingeneral, any reasonable element can be introduced as a dopant to achievedesired properties.

Suitable silicon precursors for elemental silicon particle formationinclude, for example, silane (SiH₄), disilane (Si₂H₆), trisilane(Si₃H₈), silicon tetrachloride (SiCl₄), trichlorosilane (SiCl₃H), andSiC₁₂H₂. Silane, SiH₄, is a convenient precursor for laser pyrolysissince it absorbs infrared light from a CO₂ laser and decomposes to formcrystalline silicon particles upon decomposition. The higher ordersilanes similarly decompose to form elemental silicon, i.e. Si⁰, siliconin its elemental state. Thus, with silane as a precursor, a secondaryreactant source may not be used, and a separate infrared absorber is notneeded. Corresponding germanes (GeH₄ and Ge₂H₆) can be used asprecursors. An inert gas can be used to moderate the reaction. Suitableinert gases include for example, Ar, He N₂ or mixtures thereof.

Suitable precursors for B, Al, Ga, P, As, Sb and other dopants aredescribed explicitly in the '334 patent. Suitable precursors for aerosoldelivery of gallium include, for example, gallium nitrate (Ga(NO₃)₃).Arsenic precursors include, for example, AsCl₃, which is suitable forvapor delivery, and As₂O₅, which is suitable for aerosol precursordelivery in aqueous or alcohol solutions. Suitable boron precursorscomprise, for example, diborane (B₂H₆), BH₃, and the like, and suitablecombinations of any two or more thereof. Suitable aluminum precursorsinclude, for example, aluminum hydride (AlH₃), aluminum s-butoxide(Al(OC₄H₉)₃), trimethyl aluminum (Al(CH₃)₃, trimethyl ammonia aluminumAl(CH₃)₃NH₃, and the like, and suitable combinations of any two or morethereof. Suitable phosphorous precursor compositions for vapor deliverycomprise, for example, phosphine (PH₃), phosphorus trichloride (PCl₃),phosphorous pentachloride (PCl₅), phosphorus oxychloride (POCl₃),P(OCH₃)₃, and the like, and suitable combinations of any two or morethereof. A suitable antimony precursor include, for example, stibine(SbH₃) and antimony trichloride (SbCl₃), which is soluble in alcohol. Insome embodiments, the silicon precursor is silane (SiH₄), the boronpercursor is diborane (B₂H₆), the phosphorous precursor is PH₃, and theoxygen source is O₂ or N₂O.

Surface Modification and Dispersion Process

The submicron silica particles generally are dispersed for furtherprocessing or use. In some embodiments, the dispersion can be furtherstabilized by surface modifying the silica particles. The surfacemodifying agents of particular interest can form chemical bonds with theparticle surface. Through appropriate selection of the dispersing liquidand the particle surface properties, stable dispersions can be formed atreasonable concentrations. The dispersions can be delivered throughsuitable coating approaches or printed with the dispersion used as anink.

The surface modification of inorganic particles, e.g., silica particles,can improve stability of the particle dispersions and provide fordispersion of the particles in a wider range of liquids and potentiallyat higher concentrations. While some surface modifiers can merely coatthe surface, improved stability of the coated particles may beaccomplished with surface modifiers that are chemically bonded to thesurface. In particular, alkoxysilanes react with silicon oxides to formSi—O—Si bonds to form a stable surface coating with the release of acorresponding compound from the displaced alkoxy silane functionalgroup. An improved surface coating can be achieved with improved —OHfunctional group coverage on the surface of the silicon oxide particles.The surface modification process can involve a switch of dispersants.For convenience of terminology, a surface modifying compound refers to acompound that adds at least 3 atoms to the particle surface when itbonds to the particle surface, to distinguish compositions, that modifythe surface of a silicon oxide particle such as through the introductionof an —OH group. In general, it is expected that the presence of adopant does not significantly alter the surface modification process orchemistry.

A range of surface modifying compounds can be used to chemically bond tothe silica particle surfaces. Suitable functional groups for bonding toinorganic particles with different compositions are described in U.S.Pat. No. 6,599,631 to Kambe et al, entitled “Polymer-Inorganic ParticleComposites,” incorporated herein by reference. Alkoxysilanes providestable bonding to silicon oxide particles. In particular,trialkoxysilanes provide very stable bonding to the particle surfacewith potentially three points of bonding. The fourth side chain of thetrialkoxysilanes provides the ability to influence the dispersabilityand other surface properties of the surface modified inorganicparticles. Specifically, the fourth side chain of the silane can beselected to improve disperability in a selected solvent and/or toprovide a reactive functional group for further processing. Similarly,polydialkoxysiloxy silanes provide stable bonding with the ability ofeach monomer unit to form two bonds to the particle. The siloxanepolymer can wrap around the particles during the bonding process. Inaddition to alkoxy silanes, chemical compounds with other functionalgroups can form bonds to silicon oxide particles. Specifically,compounds with chlorosilicate (—SiCl) groups, some amine groups,carboxylic acid groups and hydroxide groups can also bond to siliconoxide particle surfaces. Additional functional groups of these compoundscan be similarly selected to yield desirable properties for theresulting surface modified particles.

With respect to the alkoxy side chains of silanes, methoxy groups andethoxy groups have been found to be effective in reacting with inorganicoxide particle surfaces, and a range of compounds with these functionalgroups are commercially available. Suitable fourth functional groups forthe trialkoxy silanes include, for example, alkyl groups, epoxide groups(—(CH₂)_(n)CHCH₂O_(bridge)), methacryloxyalkyl (—(CH₂)_(n)OOC═CH₂),isocyanate (—(CH₂)_(n)NCO), thiol (—(CH₂)_(n)SH), acetyl(—(CH₂)_(n)OOCCH₃), hydroxybenzophenyl (—(CH₂)_(n)OC₆H₅(OH)COC₆H₅),allyl (—CH₂CH═CH₂), and phenethyl (—(CH₂)_(n)C₆H₅). In general, thesurface modifying compound can be coated at a coverage from less than amonolayer to four or more monolayers as well as values between. Theamount of coverage can be estimated based on the surface area of theparticles and the amount of compound that can be expected to pack alongthe particle surface.

One of at least two processes can be used to perform the surfacemodification. In one approach, an unstable, higher concentrationdispersion can be formed with the silica particles, and the surfacemodification is performed to stabilize the higher concentrationdispersion. However, better particle dispersions generally are obtainedthrough first forming a dilute, relatively stabile dispersion of theparticles without surface modification and then performing the surfacemodification.

In the direct approach, the liquid is selected to balance the dispersionof the unmodified particles, the solubility of the surface modifyingcompound unbound to the particles and the dispersion of the particlesfollowing surface modification. Generally, the liquid is not aparticularly good dispersant for the unmodified particles. Similarly,the liquid may not be a good solvent for the surface modifying agent.But if the surface modifying agent is somewhat soluble in the liquid andthe unmodified particles can be dispersed with mixing, the surfacemodification reaction can be performed. As the particles become surfacemodified, the dispersion may stabilize as the reaction proceeds.

Better dispersion results generally can be obtained if the inorganicparticles without a surface modifier are first stably dispersed with adesirably small average secondary particle size. Alcohols andwater/alcohol blends generally are good dispersants for the unmodifiedsilica particles as synthesized by some approaches. The surfacemodifying compound can be added directly into the alcohol orwater/alcohol blend if it has some solubility, or the surfacemodification compound can be dissolved into a solvent that is misciblewith or soluble in the liquid of the particle dispersion. After thesurface modification is complete, the particles can be transferred to adifferent dispersing liquid as described below. The surface modifiedparticles can be stored or shipped in a liquid suitable for furtherprocessing.

In general, to change dispersing liquids, it has been found effective tosettle the particles by forming a liquid mixture in which the stabilityof the dispersion is lost. Centrifugation or filtration can be used toefficiently separate the particles from the liquid once they are nolonger stably dispersed. If the particles are centrifuged, the liquid isdecanted from the precipitated particles. The particles can be washedone or more times with a selected dispersing liquid to remove residualamounts of the original liquid. Then, the particles can be redispersedin the selected liquid. In this way, the liquid can be changed for alater processing step through the selection of a surface modifier thatfacilitates dispersion in the selected liquid.

Following surface modification and/or at other stages of the dispersionprocess, the dispersion can be filtered to remove contaminants and orany stray unusually large particles. Generally, the filter is selectedto exclude particulates that are much larger than the average secondaryparticle size so that the filtration process can be performed in areasonable way. In general, the filtration processes have not beensuitable for overall improvement of the dispersion quality. Suitablecommercial filters are available, and can be selected based on thedispersion qualities and volumes.

Properties and Formation of the Dispersions and Inks

The dispersions can be formulated for a selected application. Thedispersions can be characterized with respect to composition as well asthe characterization of the particles within the dispersions. Ingeneral, the term ink is used to describe a dispersion that issubsequently deposited using a printing technique, and an ink may or maynot include additional additives to modify the ink properties.

The dispersions comprise a liquid and the dispersed silica particles,which may or may not be surface modified. In general, silica particlesformed by laser pyrolysis can be well dispersed in water or alcohols atmoderate concentrations with no surface modification, although higherconcentration dispersions generally can be formed with surfacemodification. Suitable alcohols include, for example, small aliphaticalcohols, such as methanol, ethanol, propylene glycol, butanediol,mixtures thereof and the like. Upon surface modification, the silicaparticles can be dispersed in a broader range of solvents and solventblends through the matching of the chemical properties of the surfacemodifying agent with the liquid. Thus, following surface modification,the particles can be well dispersed in a range of less polar solvents,such as ethyl lactate, n-methylpyrrolidinone, gamma-butyl lactone, andthe like.

Better dispersions are more stable and/or have a smaller secondaryparticle size indicating less agglomeration. As used herein, stabledispersions have no settling without mixing after one hour. In someembodiments, the dispersions exhibit no settling of particles withoutmixing after one day and in further embodiments after one week, and inadditional embodiments after one month. In general, dispersions withwell dispersed particles can be formed at concentrations of at least upto 30 weight percent inorganic particles. Generally, for someembodiments it is desirable to have dispersions with a particleconcentration of at least about 0.05 weight percent, in otherembodiments at least about 0.25 weight percent, in additionalembodiments from about 0.5 weight percent to about 30 weight percent andin further embodiments from about 1 weight percent to about 20 weightpercent. A person of ordinary skill in the art will recognize thatadditional ranges of stability times and concentrations within theexplicit ranges above are contemplated and are within the presentdisclosure.

The dispersions can include additional compositions besides the silicaparticles and the dispersing liquid or liquid blend to modify theproperties of the dispersion to facilitate the particular application.For example, property modifiers can be added to the dispersion tofacilitate the deposition process. Surfactants can be effectively addedto the dispersion to influence the properties of the dispersion.

In general, cationic, anionic, zwitter-ionic and nonionic surfactantscan be helpful in particular applications. In some applications, thesurfactant further stabilizes the particle dispersions. For theseapplications, the selection of the surfactant can be influenced by theparticular dispersing liquid as well as the properties of the particlesurfaces. In general, surfactants are known in the art. Furthermore, thesurfactants can be selected to influence the wetting or beading of thedispersion/ink onto the substrate surface following deposition of thedispersion. In some applications, it may be desirable for the dispersionto wet the surface, while in other applications it may be desirable forthe dispersion to bead on the surface. The surface tension on theparticular surface is influenced by the surfactant. Also, blends ofsurfactants can be helpful to combine the desired features of differentsurfactants, such as improve the dispersion stability and obtainingdesired wetting properties following deposition. In some embodiments,the dispersions can have surfactant concentrations from about 0.01 toabout 5 weight percent, and in further embodiments from about 0.02 toabout 3 weight percent.

The use of non-ionic surfactants in printer inks is described further inU.S. Pat. No. 6,821,329 to Choy, entitled “Ink Compositions and Methodsof Ink-Jet Printing on Hydrophobic Media,” incorporated herein byreference. Suitable non-ionic surfactants described in this referenceinclude, for example, organo-silicone surfactants, such as SILWET™surfactants from Crompton Corp., polyethylene oxides, alkyl polyethyleneoxides, other polyethylene oxide derivatives, some of which are soldunder the trade names, TERGITOL™, BRIJ™, TRITON™, PLURONIC™, PLURAFAC™,IGEPALE™, and SULFYNOL™ from commercial manufacturers Union CarbideCorp., ICI Group, Rhone-Poulenc Co., Rhom & Haas Co., BASF Group and AirProducts Inc. Other nonionic surfactants include MACKAM™ octylaminechloroacetic adducts from McIntyre Group and FLUORAD™ fluorosurfactantsfrom 3M. The use of cationic surfactants and anionic surfactants forprinting inks is described in U.S. Pat. No. 6,793,724 to Satoh et al.,entitled “Ink for Ink-Jet Recording and Color Ink Set,” incorporatedherein by reference. This patent describes examples of anionicsurfactants such as polyoxyethylene alkyl ether sulfate salt andpolyoxyalkyl ether phosphate salt, and examples of cationic surfactants,such as quaternary ammonium salts.

Viscosity modifiers can be added to alter the viscosity of thedispersions. Suitable viscosity modifiers include, for example solublepolymers, such as polyacrylic acid, polyvinyl pyrrolidone and polyvinylalcohol. Other potential additives include, for example, pH adjustingagents, antioxidants, UV absorbers, antiseptic agents and the like.These additional additives are generally present in amounts of no morethan about 5 weight percent. A person of ordinary skill in the art willrecognize that additional ranges of surfactant and additiveconcentrations within the explicit ranges herein are contemplated andare within the present disclosure.

For electronic applications and some optical application, it can bedesirability to remove organic components to the ink prior to or duringcertain processing steps such that the product materials are effectivelyfree from carbon. In general, organic liquids can be evaporated toremove them from the deposited material. However, surfactants, surfacemodifying agents and other property modifiers may not be removablethrough evaporation, although they can be removed through heating atmoderate temperature in an oxygen atmosphere to combust the organicmaterials.

The use and removal of surfactants for forming metal oxide powders isU.S. Pat. No. 6,752,979 to Talbot et al., entitled “Production of MetalOxide Particles with Nano-Sized Grains,” incorporated herein byreference. The '979 patent teaches suitable non-ionic surfactants,cationic surfactants, anionic surfactants and zwitter-ionic surfactants.The removal of the surfactants involves heating of the surfactants tomoderate temperatures, such as to 200° C. in an oxygen atmosphere tocombust the surfactant. Other organic additives generally can becombusted for removal analogously with the surfactants. If the substratesurface is sensitive to oxidation during the combustion process, areducing step can be used following the combustion to return the surfaceto its original state.

In general, if processed appropriately, for dispersions with welldispersed particles, the average secondary particle size can be no morethan a factor of four times the average primary particle size, infurther embodiments no more than about 3 times the average primaryparticle size and in additional embodiments no more than about 2 timesthe average primary particle size. In some embodiments, thevolume-average particle size is no more than about 1 micron, in furtherembodiments no more than about 250 nm, in additional embodiments no morethan about 100 nm, in other embodiments no more than about 75 nm and insome embodiments from about 5 nm to about 50 nm. With respect to theparticle size distribution, in some embodiments, essentially all of thesecondary particles can have a size no more than 5 times thevolume-average secondary particle size, in further embodiments no morethan about 4 times the volume-average particle size and in additionalembodiments no more than about 3 times the volume-average particle size.Furthermore, the DLS particle size distribution by volume can have insome embodiments a full width at half-height of no more than about 50percent of the Z-average particle size. Also, the secondary particlescan have a distribution in sizes such that at least about 95 percent ofthe particles have a diameter greater than about 40 percent of theaverage particle size and less than about 250 percent of the averageparticle size. In further embodiments, the secondary particles can havea distribution of particle sizes such that at least about 95 percent ofthe particles have a particle size greater than about 60 percent of theaverage particle size and less than about 200 percent of the averageparticle size. A person of ordinary skill in the art will recognize thatadditional ranges of particle sizes and distributions within theexplicit ranges above are contemplated and are within the presentdisclosure.

Z-average particle sizes can be measured using dynamic light scattering.The Z-average particle size is based on a scattering intensity weighteddistribution as a function of particle size. Evaluation of thisdistribution is prescribed in ISO International Standard 13321, Methodsfor Determination of Particle Size Distribution Part 8: PhotonCorrelation Spectroscopy, 1996. The Z-average distributions are based ona single exponential fit to time correlation functions. However, smallparticles scatter light with less intensity relative to their volumecontribution to the dispersion. The intensity weighted distribution canbe converted to a volume-weighted distribution that is perhaps moreconceptually relevant for evaluating the properties of a dispersion. Fornanoscale particles, the volume-based distribution can be evaluated fromthe intensity distribution using Mie Theory. The volume-average particlesize can be evaluated from the volume-based particle size distribution.Further description of the manipulation of the secondary particle sizedistributions can be found in Malvern Instruments—DLS Technical NoteMRK656-01, incorporated herein by reference.

The viscosity of the dispersion/ink is dependent on the silica particleconcentration as well as the other additives. Thus, there are severalparameters that provide for adjustment of the viscosity. The viscosityis particularly relevant for inkjet printing although other printing andcoating processes may have desired viscosity ranges. For someembodiments, the viscosity can be from 0.1 mPa·s to about 100 mPa·s andin further embodiments from about 0.5 mPa·s to about 25 mPa·s. For someembodiments, the dispersions/inks can have a surface tension from about2.0 to about 6.0 N/m² and in further embodiments from about 2.2 to about5.0 N/m² and in additional embodiments form about 2.5 to about 4.5 N/m².A person of ordinary skill in the art will recognize that additionalranges of viscosity and surface tension within the explicit ranges aboveare contemplated and are within the present disclosure.

The dispersions/inks can be formed using the application of appropriatemixing conditions. For example, mixers/blenders that apply shear can beused and/or sonication can be used to mix the dispersions. Theparticular additives can be added in an appropriate order to maintainthe stability of the particle dispersion. In general, appropriatelyselected surfactants and some property modifiers can stabilize theparticle dispersion. A person of ordinary skill in the art can selectthe additives and mixing conditions empirically based on the teachingsherein.

Printing and Other Deposition Approaches

The dispersions/inks can be deposited for using a selected approach thatachieves a desired distribution of the dispersion on a substrate. Forexample, coating and printing techniques can be used to apply the ink toa surface. Using selected printing approaches, patterns can be formedwith high resolution. Following deposition, the deposited material canbe further processed into a desired device or state.

Suitable coating approaches for the application of the dispersionsinclude, for example, spin coatings, dip coating, spray coating,knife-edge coating, extrusion or the like. Coating approaches generallyare used to cover a substrate, although a mask or the like can be usedto limit the deposition locations following removal of the mask. Ingeneral, any suitable coating thickness can be applied, although inembodiments of particular interest, coating thickness can range fromabout 50 nm to about 500 microns and in further embodiments from about100 nm to about 250 microns. A person of ordinary skill in the art willrecognize that additional ranges of thicknesses within the particularranges above are contemplated and are within the present disclosure.

Similarly, a range of printing techniques can be used to print thedispersion/ink into a pattern on a substrate. Suitable printingtechniques include, for example, screen printing, inkjet printing,lithographic printing, gravure printing and the like. Suitablesubstrates include, for example, polymers, such as polysiloxanes,polyamides, polyimides, polyethylenes, polycarbonates, polyesters,combinations thereof, and the like, ceramic substrates, such as silicaglass, and semiconductor substrates, such as silicon or germaniumsubstrates. The composition of the substrates influences the appropriaterange of processing options following deposition of the dispersion/ink.

While various coating and printing approaches are suitable, inkjetprinting offers desirable features with respect to speed, resolution andversatility with respect to real time selection of deposition patterningwhile maintaining speed and resolution. However, practical depositionusing inkjet printing with inorganic particles requires dispersionproperties that have not been available prior to development of both thetechniques to form high quality silica nanoparticle along with theimproved ability to form high quality dispersions from these particles.Thus, the particles produced using laser pyrolysis combined with theimproved surface modification approaches and dispersion techniquesprovides for the formation of inks that are amenable to inkjetdeposition.

In general, following deposition, the liquid evaporates to leave theparticles and any other non-volatile components of the inks remaining.If the substrate tolerates suitable temperatures and if the additiveshave been properly selected, the additives can be removed through theaddition of heat in an appropriate oxygen atmosphere to remove theadditives, as described above. Similarly, with suitable substrates, thesilica particles can then be melted to form a cohesive mass of thesilica deposited at the selected locations. If the heat treatment isperformed with reasonable control over the conditions, the depositedmass does not migrate significantly from the deposition location, andthe fused mass can be further processed into a desired device.

Solar Cell and Other Semiconductor Applications

For solar cell and other semiconductor applications, silica particlescan be used to form dielectric components and/or deliver dopants tosemiconductor substrates, such as silicon and/or germanium materials. Arepresentative printed substrate is shown in FIG. 1. In this embodiment,substrate 100 has a surface coating 102 with windows 104, 106 throughcoating 102 to expose a portion of the substrate surface. Silica ink isprinted to form deposits 108, 110 on the substrate surface. Thesubstrate comprises silicon, germanium or an alloy thereof. Suitablesubstrates include, for example, high purity silicon wafers and thelike. In other embodiments, suitable substrates includesilicon/germanium foils, as described in copending U.S. patentapplication Ser. No. 11/717,605 filed on Mar. 13, 2007 to Hieslmair etal., entitled “Thin Silicon or Germanium Sheets and Photovoltaics FormedFrom Thin Sheets,” incorporated herein by reference. A silicadispersion/ink can be applied over the surface using the coating orprinting techniques described above.

Some specific embodiments of photovoltaic cells using thin semiconductorfoils and back surface contact processing is described further in U.S.Provisional patent application Ser. No. 60/902,006 filed on Feb. 16,2007 to Hieslmair, entitled “Photovoltaic Cell Structures, Solar Panels,and Corresponding Processes,” incorporated herein by reference. In someembodiments, the silica ink is applied through pre-established windowsto the substrate surface. The doped ink is printed within the substratewindows. Other patterning or no patterning can be used as desired for aparticular application.

To deliver dopant into the semiconductor from the silica deposit, thematerial is heated. For example, the structure can be placed into anoven or the like with the temperature set to soften the particles suchthat the dopants can diffuse into the substrate. The time andtemperature can be adjusted to yield a desired dopant migration into thesubstrate. Alternative approaches can be used to heat the surface of thesubstrate with the deposit. A rapid thermal anneal for the general drivein of a dopant into a semiconductor material is described further inU.S. Pat. No. 6,287,925 to Yu, entitled “Formation of Highly ConductiveJunctions by Rapid Thermal Anneal and Laser Thermal Process,”incorporated herein by reference. The material can be heated to atemperature of under 1000° C. for an appropriate period of time to allowfor dopant migration.

However, improved control of the resulting doped substrate as well asenergy saving can be obtained through the use of UV light to melt justthe surface of the substrate without generally heating the substrate oronly heating the substrate to lower temperatures. Local hightemperatures on the order of 1400° C. can be reached to melt the surfacelayer of the substrate as well as the oxide particles on the substrate.Generally, any intense UV source can be used, although excimer lasers orother lasers are a convenient UV source for this purpose. Eximer laserscan be pulsed at 10 to 300 nanoseconds at high fluence to briefly melt athin layer, such as 20 nm to 1000 nm, of the substrate. Longerwavelength light sources such as 1 micron wavelength light sources canalso be used.

Following the drive in of the dopant, it may or may not be desirable toremove the silica. For photovoltaic applications it is generallydesirable to remove the silica to expose the doped semiconductor for theapplication of conductive electrical contacts. To remove the silica, theoxide can be etched, for example, using conventional approaches, such asusing wet (chemical) etching or plasma etching. Suitable chemicaletching compositions include, for example, tetramethyl ammoniumhydroxide solution, or HF. Once the oxide is removed, the substrate canbe further processed.

These same heating approaches can be used to process the silicaparticles into a solid dielectric mass. The particles may or may not bedoped for these embodiments.

Optical Applications

For optical applications, generally the deposited silica is melted orsintered to form a solid mass from the particles to reduce scattering atthe particle surfaces. Dopants can be introduced to alter the processingproperties, such as the softening temperature for the particles, or theoptical properties, such as the index-of-refraction or optical emissionproperties. The approaches described above for semiconductor processingcan be adapted for optical applications. In general, for opticalapplications, the silica inks can be deposited to form particularoptical structures, such as display elements or waveguides.

For example, in some embodiments, the inks can be printed at particularlocations along a structure for the formation of a display. Then, thedeposits can be cured at the printed locations. The dopants can bevaried to alter the optical properties at different locations along thedisplay. In some embodiments, the particles can comprise phosphormaterials coated with silica. The coating of phosphor particles withsilica is described further in U.S. Pat. No. 5,908,698 to Budd, entitled“Encapsulated Electroluminescent Phosphor and Method for Making Same,”and in U.S. Pat. No. 5,599,529 to Cowie, entitled “Dispersions,” both ofwhich are incorporated herein by reference. The coated particles can bedispersed and deposited according to the description of other silicaparticles described herein. The core phosphor particles can be formed,for example, as described in U.S. Pat. No. 6,692,660 to Kumar, entitled“High Luminescent Phosphor Particles and Related Particle Compositions,”incorporated herein by reference. Different phosphor materials can bedeposited at different locations to form a desired display device. Whilemany display structures are known or can be subsequently developed inthe art, and example of a display structure is found in U.S. Pat. No.5,651,712, entitled “Multi-Chromic Lateral Field Emission Devices WithAssociated Displays and Methods of Fabrication,” incorporated herein byreference.

Waveguides can be used to transmit optical signals, for example, inconnection with telecommunication signals. Waveguides can take the formof optical fibers, planar waveguides or the like. The printingapproaches described herein can be used to form optical waveguidesthrough the deposition of silica particle dispersions and the subsequentfusing of the particles into an optical structure. The approachesdescribed herein are alternatives to chemical vapor deposition combinedwith photolithography and to Light Reactive Deposition (LRD™), asdescribed in WO 02/32588 to Bi et al., entitled “Coating Formation byReactive Deposition,” incorporated herein by reference. The depositionapproaches described herein can be used to form the waveguide cores thatcan be covered with a lower index-of-refraction cladding material, forexample, using chemical vapor deposition or LRD™. With respect toalternative approaches to planar waveguide formation, the formation ofplanar waveguides by flame hydrolysis is described in U.S. Pat. No.3,934,061 to Keck et al., entitled “Method of Forming Planar OpticalWaveguides,” incorporated herein by reference.

EXAMPLES Example 1 SiO₂ Dispersion in Methyl Ethyl Ketone (MEK)

This example demonstrates the dispersion and surface modification ofsilica particles synthesized using laser pyrolysis.

SiO₂ was produced by laser pyrolysis aerosol) with Tetraethoxylsilane(TEOS) as the silicon precursor. The silicon precursor was deliveredwith as an aerosol with a mixture of ethylene, oxygen and an inert gasserving as the carrier gas. The apparatus was similar to the apparatusin FIGS. 6-8 of U.S. Pat. No. 6,849,334 to Home et al., entitled“Optical Materials and Optical Devices,” incorporated herein byreference. The SiO₂ particles had a primary particle size as determinedusing a transmission electron micrograph of 10-20 nm. Transmissionelectron micrographs at two magnifications are shown in FIGS. 2 and 3,respectively, for a representative SiO₂ nanoparticle sample made fromTEOS using laser pyrolysis.

SiO₂ powder (13.6 g, with a BET surface area>236.5 m²/g) was added tomethylethylketone (MEK) to make a 5% wt mixture. The mixture wassonicated with a probe sonicator for 1-4 hours. Then, methacryloxypropyltrimethoxysilane (Z6030, 3.0 g-17.9 g, Dow Corning.) was added to thedispersion, and sonication was continued for 1-3 hr. The resultantsecondary particles sizes were evaluated with dynamic light scattering(DLS) using a Malvern ZetaSizer™ instrument. The distribution of thesize of the particles revealed from DLS measurement is shown in FIG. 4where about 66% of the secondary particles had a diameter of 362.6 nmand 34% of the particles had a peak secondary particle size of about28.1 nm. SiO₂ particles therefore form a good dispersion in MEK at 5% wtafter modification with methacryloxypropyl trimethoxysilane.

Example 2 SiO₂ Dispersion in Isopropanol

This example demonstrated the dispersion of SiO₂ nanoparticles inisopropanol as an alternative dispersing liquid. The SiO₂ nanoparticlesfor this example had the same property as the nanoparticles in Example1.

SiO₂ powder (1.0 g, with a BET surface area 214.0 m²/g) was added toisopropanol to make a 1% wt mixture. The mixture was sonicated in a bathsonicator for 1-4 hours. Then,N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (0.32 g-1.94 g, Gelest,Inc.) was added to the dispersion and continue sonication for 1-7 hr ata temperature of 25° C.-60° C. The resultant secondary particles sizeswere evaluated with dynamic light scattering (DLS) using a MalvernZetaSizer™ instrument. The distribution of the size of the particlesrevealed from DLS measurement is shown in FIG. 5 where the particles hadan average secondary particle size of about 99.7 nm. SiO₂ particlestherefore form good dispersion in IPA at 1% wt after modification withN-(2-aminoethyl)-3-aminopropyltrimethoxysilane methacryloxypropyltrimethoxysilane.

The embodiments above are intended to be illustrative and not limiting.Additional embodiments are within the claims. In addition, although thepresent invention has been described with reference to particularembodiments, those skilled in the art will recognize that changes can bemade in form and detail without departing from the spirit and scope ofthe invention. Any incorporation by reference of documents above islimited such that no subject matter is incorporated that is contrary tothe explicit disclosure herein.

1. A dispersion comprising a liquid and doped silica/germaniananoparticles, the doped silica/germania nanoparticles having an averageprimary particle size from about 1 nm to about 100 nm and avolume-average particle size of no more than about 500 nm, thedispersion having a concentration of doped silica/germania nanoparticlefrom about 0.1 weight percent to about 20 weight percent dopedsilica/germania nanoparticles, wherein the silica nanoparticles comprisea dopant element from about 1.0×10⁻⁷ to about 15 atomic percent relativeto the silicon/germanium atoms of the silica/germania nanoparticles. 2.The dispersion of claim 1 wherein the volume-average particle size is nomore than about 250 nm.
 3. The dispersion of claim 1 wherein theconcentration of doped silica/germania nanoparticles is from about1.0×10⁻⁵ weight percent to about 1 weight percent.
 4. The dispersion ofclaim 1 wherein the secondary particle size distribution has a majoritypeak with a full width at half maximum of no more than about 50% of thevolume-average secondary particle size.
 5. The dispersion of claim 1wherein the collection of silica/germania particles have an averageprimary particle size from about 1 nm to about 100 nm.
 6. The dispersionof claim 1 wherein the silica/germania particle have a surfacemodification composition selected to be compatible with the liquid.
 7. Amethod for forming a dispersion, the method comprising blending acollection of doped silica/germania nanoparticles with a liquid whereinthe liquid and nanoparticles are selected to be compatible with eachother, the nanoparticles having an average primary particle size fromabout 1 nm to about 100 nm and a volume-average secondary particle sizeno more than about 500 nm following mixing, wherein the silica/germaniananoparticles comprise a dopant element from about 1.0×10⁻⁷ to about 15atomic percent relative to the silicon/germanium atoms of thesilica/germania nanoparticles the formed dispersion having aconcentration of doped silica/germania nanoparticle from about 0.1weight percent to about 20 weight percent doped silica/germaniananoparticles.
 8. The method of claim 7 wherein the blending comprisessonication.
 9. The method of claim 7 wherein the blending comprises theapplication of shear using a mixing element.
 10. The method of claim 7wherein the silica/germania particles have a surface modifyingcomposition selected to be compatible with the liquid.
 11. A method fordoping a semiconductor substrate comprising silicon or germanium, themethod comprising heating a coating in contact with substrate to drivedopant into the substrate, wherein the coating comprises dopedsilica/germania particles, the doped silica/germania particlescomprising a dopant element from about 1.0×10⁻⁷ to about 15 atomicpercent relative to the silicon/germanium atoms of the silica/germaniaparticles, the particles having an average primary particle size fromabout 1 nm to about 100 nm and an average second particle size no morethan about 500 nm based on a dispersion formed with the particles priorto forming a coating said dispersion having a concentration of dopedsilica/germania nanoparticle from about 0.1 weight percent to about 20weight percent doped silica/germania nanoparticles.
 12. The method ofclaim 11 wherein the heating is performed with a laser.
 13. The methodof claim 11 wherein the heating is performed with a light source. 14.The method of claim 11 wherein the heating is performed in an oven. 15.The method of claim 11 further comprising removing at least some of thesilica/germania following the driving of the dopant into the substrate.